1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method. In particular, the present invention relates to an application of a smoothing processing to an image and manner of the application.
2. Description of the Related Art
Recently, multi function machines including an ink jet print type printing mechanism and a scanner mechanism (hereinafter also referred to as IJMFP) have been widely provided. IJMFP is a printer that can be connected to a personal computer (PC) to provide various applications such as a function to perform printing and scanning, a copy function of the machine itself, and a function by which the machine is connected to a digital camera for direct printing. Thus, the multi function machines have been used as a copying machine for family use for example. The copy function of IJMFP reads a document image by a scanner mechanism to print the image on a printing medium such as a paper.
In the copying as described above, the color reproduction range and the like is generally different depending on the type of a document to be copied. Thus, there may be a case where the document and a copy output have difficulty in having identical visual colors. There may be another case where different gradation characteristics may be reproduced depending on the type of a document.
Difference in document is caused, for example, between when a print printed by a printer is copied and when a silver halide photograph is copied. Specifically, a printer performs are production of an image by an area coverage modulation in which printing is performed on a printing medium based on quantized data to provide the print whose gradation is expressed by the area coverage modulation such as a halftone dot document. On the other hand, silver halide photography performs a reproduction of an image by a density modulation in which gradation is expressed by the density itself. If a read image is subjected to a uniform processing and printing is performed based on the uniform processing result regardless of whether the document is based on the area coverage modulation or the density modulation, a problem may be caused where how a base color changes or a color is reproduced is different depending on the type of a document.
To solve this problem, Japanese Patent Laid-Open No. 2001-251513 proposes a technique using image region segmentation. Specifically, this publication describes a method for segmenting a read image to at least a halftone dot region and a photo region and subjecting the respective regions to optimal γ conversions to provide favorable images in all regions. Also, Japanese Patent Laid-Open No. 2002-218271 describes a method for similarly segmenting a read image to a character region and a photograph region and subjecting the respective regions to optimal color space conversions to provide favorable images in all regions.
However, in the case of the above-described methods of Japanese Patent Laid-Open No. 2001-251513 and Japanese Patent Laid-Open No. 2002-218271, the respective regions subjected to the image region segmentation are subjected to optimal color conversions. Thus, for each of the segmented regions, a color design must be performed in accordance with the output characteristic of the printer. In this case, an accurate color reproduction requires increased types of regions subjected to the image region segmentation. For example, halftone dot regions require different output characteristics of the printer depending on the number of lines and a screen angle. Thus, regions must be segmented to the number of regions in accordance with these printer output characteristics. Because of this, the color conversion processing for the respective regions requires an increased amount of processing to consequently require a significant amount of memory.
In the meantime, smoothing processing can basically provide an effective measure against the above-described problem where a different color or gradation is reproduced dependent on the type of the document. Specifically, an image may be subjected to the smoothing processing to provide, in the subsequent color conversion processing, a certain color or gradation, which is shown in the image finally printed based on the reading result, regardless of the type of the document. The reason will be briefly described below.
FIG. 1 is a graph showing color differences, as the maximum color difference and an average color difference, between an image printed without subjecting an image read by a scanner to the smoothing processing and an image printed with subjecting the read image to the smoothing processing. In particular, FIG. 1 shows resultant color differences between respective sets of 729 color patches each set of which is made by combining respective nine levels of colors of R, G, and B and which are printed by respective three types of print methods of the offset printing, the silver halide photography, and the ink jet photograph printing (hereinafter IJ photograph printing). As can be seen from FIG. 1, those subjected to the smoothing processing and those not subjected to the smoothing processing have certain color differences therebetween respectively. In particular, the offset printing shows significant differences between those subjected to the smoothing processing and those not subjected to the smoothing processing, and also shows a higher average color difference than the maximum color differences of the silver halide photograph and the IJ photograph.
FIGS. 2A to 2D are diagrams illustrating how the smoothing processing reduce the influence by the color difference as described above on the reproduced color or the reproduced gradation characteristic.
FIG. 2A schematically illustrates a document image represented based on the area coverage modulation. FIG. 2B schematically illustrates the document image based on the density modulation. The image based on the area coverage modulation shown in FIG. 2A consists of three colors of colors 21A, 21B, and 21C and is macroscopically recognized as having the same colors of the image based on the density modulation shown in FIG. 2B. For example, a color conversion processing for print output such as an output device color conversion, which will be described later, is applied to each pixel and the color conversion is optimally controlled for each pixel. FIG. 2C and FIG. 2D show the result of subjecting the images shown in FIG. 2A and FIG. 2B to the color conversion processing for print output, respectively. The colors 23A, 23B, and 23C in FIG. 2C show the result of subjecting the pixels of the three colors of 21A, 21B, and 21C to the color conversion processing, respectively. When the three colors of 21A, 21B, and 21C are close to one another, the image shown in FIG. 2C and the image shown in FIG. 2D, which have been subjected to the color conversion for print output, have macroscopically close colors to each other. However, when the three colors of 21A, 21B, and 21C are distant from one another, the image shown in FIG. 2C and the image shown in FIG. 2D, which have been subjected to the color conversion, have macroscopically distant colors from one another.
Consequently, the image shown in FIG. 2A is previously subjected to the smoothing processing to cause the colors 21A, 21B, and 21C to be close to each other so that the image shown in FIG. 2C and the image shown in FIG. 2D, which have been subjected to the color conversion for print output, have macroscopically close colors to each other. In this manner, the smoothing processing can reduce the problem caused by different types of manuscript copies that cause proportionally-different reproduced colors or reproduced gradation characteristics. The smoothing is effective for a document causing a large color difference such as the document printed by the offset printing because the effect of the averaging effect by the smoothing processing is proportionally high.
However, when the smoothing processing described above is simply applied, the gradation characteristic of the printed image may be degraded in the relation with the color conversion for print output that is executed after smoothing processing.
For example, in printing considering a copy function in particular, a gamut mapping, which is a color conversion for print output, may reproduce colors having the lightness equal to or higher than the lightness of the white point of a printer color gamut as white of the printer color gamut and colors having the lightness equal to or lower than the black point of the printer color gamut as black of the printer color gamut. These methods are also called as “whitening” and “blacking”, respectively. These methods can realize favorable image reproduction even when copy is repeated for the document which has been obtained by the copy. However, if the smoothing processing is uniformly applied to the gamut of image data, pixels in a certain region may be entirely subjected to the blacking or whitening in the subsequent gamut mapping. In this case, this region is entirely represented by black or white, damaging the gradation characteristic of the entire image.
In compressing the color gamut in the gamut mapping, colors to be compressed is also subjected to the smoothing processing to cause colors having originally-different gradation values to be mapped to the same color. As a result, a gradation characteristic of an image may be damaged.